This invention relates generally to variable air volume air distribution systems for delivering conditioned air through supply air ducts to an area or space in response to a sensed condition, and more particularly, to a control for determining and maintaining a desired flow of conditioned air into a space irrespective of the pressure in a supply air duct.
The utilization of variable air volume (VAV) air distribution systems to supply conditioned air from a central source thereof to offices, school rooms, and other similar spaces or areas in multi-room buildings has become increasingly more prevalent. Such VAV systems generally furnish constant temperature air from an outlet into the space, while varying the volume of air furnished into a space in accordance with the zone temperature demands. The flow of conditioned air from the outlets is generally regulated by operation of suitable damper means controlled by a thermostat sensing the temperature of the space being conditioned. Thus as the temperature of the space deviates to a greater degree from a predetermined set point, a greater quantity of conditioned air is discharged into the space. Conversely, when the temperature in the space being conditioned approaches the set point, the quantity of conditioned air discharged into the space is reduced. Thus, the system varies the air volume to the space depending upon the deviation of the actual space temperature from the desired set point. In U.S. Pat. No. 4,756,474 assigned to the same assignee as the present invention there is described a controller for a duct pressure power air terminal unit having a volume controller which receives two pressure signals, whereby the controller bleeds one pressure signal so as to control the inflation of a bellows to thereby modulate the terminal unit to maintain a constant volume air flow through the unit. The controller bleeds the second pressure signal so as to maintain at least a minimum flow through the unit. The above-identified controller is an improvement over U.S. Pat. No. 4,120,453 which describes a three-way valve controller having two pressure regulators and a bleed type thermostat which provide four input signals to the three-way valve thereby providing a single pressure signal to the inflatable bellows.
Since the air terminals in the aforementioned patents are powered by static pressure in the duct and a continuous fill and bleed of the bladder to maintain a desired space temperature, the static pressure in the duct is continually changing as the bladder fills and bleeds and changes the flow through the terminal. Further, under normal operating conditions the air terminals closer to the central source have a higher static pressure than air terminals more distant from the central source. These differences in duct static pressure affect the rate of change of the damper. Since, these units are designed to have a single optimum rate of change in flow for control and since the static pressure within the duct changes they become under-damped and unstable at high duct pressures and become over-damped and unstable at low duct pressures.
Thus there is a clear need for a method and apparatus to maintain stable control of a damper at all duct pressures. However, no previously proposed air distributing unit controllers have incorporated any means to ensure efficient steady state operation of the unit while delivering the correct amount of conditioned air with respect to a desired air flow.